IMPACT OF FINITE VEGETATION PATCHES ON FLOW AND BED MORPHOLOGY IN OPEN CHANNELS 開水路における有限幅植生帯が水流と河床変動に及ぼす影響に関する研究
IMPACT OF FINITE VEGETATION PATCHES ON FLOW AND BED MORPHOLOGY IN OPEN CHANNELS
For the planting management and stream restoration, isolated patches of vegetationare practical for improving stream habitat and ecology. Comprehensive understandingsof flows and bed morphological changes with finite patches of vegetation are of highinterest to practitioners and engineers to design stream restorations. This study is toinvestigate morphological responses to finite patches of vegetation in open channels andunderstand the sediment transport and scour processes around vegetation by means oflaboratory experiments and numerical computations.Firstly, the effect of a finite patch of vegetation on flow and bed morphology in anopen channel was investigated. Laboratory experiments were carried out with a mobilebed included a finite patch placed along a side wall of the channel and variousconfigurations (patch density, submergence ratio and obstruction ratio) were considered.Two flow conditions below and above threshold of sediment motion, were tested.Erosion and deposition near the patch of vegetation were analyzed.Secondly, morphological responses to a finite patch in an open channel with erodiblebank were described by employing laboratory experiments and numerical simulations.The patch was modeled as circular cylinders with a staggered array, and it was locatedin the middle of the channel. The morphological behavior of a channel with erodiblebank was assessed. Numerical simulations were performed and the sensitivity toparameters related to sediment transport within the patch was investigated. Thesimulated results for erosion and deposition near the patch were compared withexperimental data and a good agreement was found.Thirdly, development of local scour pool produced by finite patches of vegetationwas investigated by means of laboratory experiments and numerical simulations. Twofinite patches were located along both side walls of the channel. Diverse ratio of thepatch width was considered to examine impacts of spacing between patches withvarious patch densities. Local erosion at the leading edge and between patches wasanalyzed. Numerical simulations with adopting bed correction model were performedand compared with experimental data. Numerical model employing bed correctionmodel demonstrated that the development of local scour pool near the patches can beiiiimproved.Lastly, in order to investigate local scour around cylinders, numerical simulationswere performed by large-eddy simulation (LES). The sediment was simulated asparticles in a Lagrangian framework and linked to the bed morphodynamic, which weredirectly incorporated with instantaneous flow on the bed. The numerical model wasvalidated by comparison with experimental data available in literatures. After validation,the impact of cylinder spacing on local scour and deposition patterns was assessed. Inaddition, the global and local scour around square cylinder groups was examined andthe influence of cylinder array on scour was assessed. Simulated maximum scour depthwas compared with experimental data and showed a reasonable agreement.
Hokkaido University（北海道大学）. 博士(工学)